Liquid propellant storage tank

ABSTRACT

A multi-layer insulated tank for storing a liquid propellant. A tank is coated with a low thermal conductive gas expanded foam. The foam coated tank is encased in a plastic bag and is maintained under a low thermal conductive gas until just before the tank is charged with the liquid propellant. Just prior to charging the tank with cryogen, the low thermal conductive gas is replaced by helium or nitrogen depending upon the cryogen to prevent condensation of water vapor and/or purge gas on the outer surface of the tank.

United States Patent Bancroft [54] LIQUID PROPELLANT STORAGE TANK [72]Inventor: George H. Bancroft, Davenport,

Iowa

[73] Assignee: The Bendix Corporation [22] Filed: May 14, 1970 [21]Appl. No.: 37,072

[ 51 Oct. 3, 1972 11/1961 Match ..220/9 LG l/l962 Hnilicka, Jr. ..220/10Primary Examiner-William F. ODea Assistant ExaminerRonald C. CaposselaAttorney-William N. Antonis and Flame, Hartz, Smith and ThompsonABSTRACT 8 Claims, 2 Draiving Figures 521 U.S.,Cl. ..62/45, 220/9 LG,200/9 F 51 Int. Cl. ..F17c 13/00 581 Field of Search ..62/45, 54,61 1;220/9 L/G, 220/10, 9 F

[56] References Cited UNITED STATES PATENTS 3,321,159 5/1967 Jackson..62/45 X 3,170,310 2/1965 Croneld ..62/DIG. 13

PUEGING NEH/VS 2 s 70 fl/E L f SUPPLY PATENTED w 3 I972 PURGING NEHNSFIG.

pueanvs r NEH/V5 28 T0 FUEL INVENTOR FIG. 2 BY PM,

ATTORNEY LIQUID PROPELLANT STORAGE TANK BACKGROUND OF THE INVENTIONother. In the case of space boosters and particularly for large storagetanks, the second tank wall represents a large weight penalty on thepayload of the booster. Once the tank reaches space, even with a singlewalled tank, the vacuum is no problem since it is the atmosphere ofspace thereby eliminating the transfer of heat by conduction. However,in space the transfer of heat by radiation affects the cryogens in thestorage tank. In order to reduce the transfer of heat by radiation whichis not affected by the gas pressure in the annulus between the doubletanks, discreet radiation shields or multi-layer insulation isintroduced into the annular space.

However, during the filling process and holding time on the ground, anuninsulated tank would experience high heat transfer and consequent lossof fluid due not only to the gaseous conduction and radiation, but alsoa very substantial heat loss from the action of gaseous convection. Toreduce the heat loss from such gaseous convection in large liquidpropellant storage tanks used in space boosters for the storage,primarily of cryogens such as liquid oxygen, liquid hydrogen and liquidmethane, foam insulation is used as one member of the insulation system.It has been observed that initially following the curing period, thethermal conductivity of a FREON expanded foam is lower than afterstanding for a few months exposed to air. It has been proposed that, theF REON diffuses out of the closed cells of the polyurethane foam and issubsequently replaced with air which has a gaseous thermal conductivityapproximately twice that of F REON. This replacement process startsimmediately on formation of the foam. The longer the foam is held instorage, the larger the percentage of air and the smaller the percentageof FREON becomes, with the attendant increase in thermal conductivity.

SUMMARY OF THE INVENTION The transfer of heat by gaseous convection tothe storage tank will be the greatest at the surface of the earth. In myinvention, I place a plastic membrance around the FREON expanded foamcoated storage tank to form a control chamber therein. By maintaining alow thermal conductive gas which has relatively high liquidus andsolidus temperatures in the control chamber during storage of the tankafter fabrication,

the maximum effectiveness of the foam coating in reducing heat loss willbe obtained.

In addition by using a plastic membrance as the second tank wall, asubstantial reduction in weight of the storage tank will be produced.Once the storage tank reaches space, the plastic membrane can bejetisoned or destroyed since the atmosphere of space is a vacuum.

It is therefore an object of this invention to provide a meansforreducing thermal conduction between a storage tank and anenvironmental source.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a cryogenic storage tankconstructed in accordance with the principles of the present invention.

FIG. 2 is another embodiment of the storage tank with a multilayerradiation shield surrounding the cryogenic storage tank.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS In the drawings, likemembers represent the same element in each embodiment.

The cryogenic storage container 10 shown in FIGS. 1 and 2 has an innerwall 12 surrounded by a FREON expanded polyurethane foam 14 to reducethe transfer of heat by convection to the inner wall 12 from anenvironmental source. A single layer of metalized material 13 such asMYLAR is wrapped on the external surface of the foam layer to reduce theabsorption of radiation in the outer surface of the foam. A plasticflexible membrane or bag 16 encases the foam covered inner wall 12forming a control chamber 18. The plastic membrance 16 has a suitablecontrol valve 21 for regulating fluid through inlet port 22 to controlchamber 18 and a suitable outlet valve 24 adjacent port 26, attached topurging means 28 for evacuating control chamber 18 in response to anoperator command.

To obtain the maximum effectiveness of the FREON expanded polyurethanefoam, the control chamber 18 is maintained during storage after curingunder a low thermal conductive gas atmosphere having relatively highliquidus and solidus temperatures. After control chamber 18 has beenevacuated of air by purge means 28, either FREON or carbon dioxide gasis supplied to the control chamber 18 through control valve 21 whichwill retain the FREON expanded foam in an inert condition.

This is because carbon dioxide has approximately the same gaseousthermal conductivity as the FREON which is of the order of 0.005 BTU perhour per sq. ft. per degree rankine while air has a thermal conductivityapproximately 0.0100 twice that of FREON or CO On the basis of molecularsize, the permeation or diffusion coefficient for CO of the FREONexpanded polyurethane foam should be between that for FREON and that forair, that is greater than that for FREON but less than that for air. Inthis way the foam will always have FREON or a mixture of FREON andcarbon dioxide contained in its cells. Thus, the thermal conductivity ofthe foam will be maintained relatively as low as when first formed andits degradation with time should be eliminated.

Upon charging chamber 30 with a cryogen through connecting means 32having a valve 20 controlled by an operator, the low thermal conductivegas is removed from control chamber 18 by purging means 28. Helium gasis now scheduled through control valve 21 to the control chamber 18.Just prior to launch both valves 21 and 24 should be closed trapping aslight positive pressure, above atmospheric, of helium gas in thecontrol chamber 18. The helium gas will not condense into a liquid or asolid at the charging temperature of the cryogen as would either theFREON or carbon dioxide gas and being dry, it will prevent the formationof frost due to the presence of water vapor in the atmosphere. Thethermal conductivity of helium is approximately to l 1 times that ofeither FREON or carbon dioxide. Although after an extended period oftime, the helium will diffuse into the foam, during the normal holdtime, the build-up of the partial pressure of helium in the foam cellswould be expected to be negligibly small, much less than a smallfraction of one atmosphere. The thermal conductivity of the mixture ofcarbon dioxide or FREON and helium will be only slightly more than thatof the carbon dioxide and FREON either alone or as a mixture of the two.As the temperature in the foam falls to the liquification orsolidification point of either FREON or carbon dioxide their vaporpressure is reduced and consequently the contribution to gaseousconduction from this source is also reduced.

When the foam insulated tank is projected into space, the primary heattransfer is through radiation. To reduce the effect of radiation from anenvironmental source, multi-layer radiation shields 34 (see FIG. 2) arewrapped around the FREON expanded polyurethane foam. The multi-layerradiation shields have a reflective metallic material plated 36 on abacking member such as MYLAR 38.

Since a slightly positive pressure of helium was established in thecontrol chamber 18 on the ground, once the storage tank has beenprojected into space, the helium gas will expand causing the plasticmembrance to rupture due to the increased differential pressure. Uponrupturing, the foam covered storage tank will then be in the absolutevacuum of space wherein the transfer of heat by convection is eliminatedand the multi-layer radiation shield 34 is the primary source of thermalenergy control.

Iclaim:

l. A storage unit, comprising:

a vessel;

means for charging said vessel with a liquid propellant;

a foam insulating member surrounding said vessel, said foam insulatingmember having gaseous cells formed by a low thermal conductive gas forreducing the transfer of heat to said vessel by gaseous convection;

a plastic bag encasing said foam insulating member forming a controlchamber around said vessel, said bag having an inlet port and an outletport;

evacuation means connected to said outlet port for removing the air fromsaid control chamber; and

control means regulating the flow of said low thermal conductive gashaving a relatively high liquidus and solidus temperature, through saidinlet port to said air evacuated control chamber for preventingdiffusion of said low thermal gas from said cells after curing duringthe storage period prior to said charging of the vessel.

2. A storage unit, as recited in claim 1, wherein said evacuating meanspurges said control chamber of said low thermal conductive gas and saidcontrol means regulates the flow of helium to said control chamber inresponse to an operator signal for preventing condensation of the lowthermal conductivity gas and water vapor on the outer surface of saidvessel during charging of said vessel with said liquid propellant.

3. A storage unit, as recited in claim 2, including:

a reflective metallic material plated on a backing member to form aradiation shield, said radiation shield being wrapped on the outersurface of said foam insulating member for reducing the transfer of heatto said vessel by radiation from an environmental source.

4. A method of reducing the transfer of heat between and externalsurface of a cryogenic storage tank and the environment, comprising thesteps of:

coating said external surface with a FREON expanded polyurethane foam;

surrounding said surface with a membrance to form a control chambertherein;

purging said control chamber of air;

introducing a gas into said control chamber having a relatively lowthermal conductivity and relatively high liquidus and solidustemperatures as compared to those of air for insulating said cryogenicstorage tank;

wrapping said external surface of said layer of foam on the externalsurface of said storage tank with a reflective metallic material appliedto a backing member to form an initial radiation shield; and

changing said gas to helium gas prior to charging said storage tank witha cryogen for preventing condensation of the said low thermalconductivity gas and water vapor on the outside of said storage tank.

5. In a cryogenic storage system, means for reducing the transfer ofheat to the external surface of a storage tank, comprising:

a foam coating of FREON expanded polyurethane secured to the externalsurface of said storage tank;

a flexible membrance surrounding said coating having an inlet port andan outlet port, said flexible membrane and coating forming a controlchamber therebetween; and

purging means operatively connected to said inlet port and said outletport for removing air from said control chamber and for introducing agas to said control chamber having a relatively low thermal conductivityas compared to that of air and having high liquidus and solidustemperatures relative to the usual cryogens for maintaining said FREONexpanded polyurethane in an inert condition.

6. In a cryogenic storage system, as defined in claim 5, wherein purgingmeans replaces said gas in said control chamber with helium prior tocharging said storage tank with cryogen for preventing consensation ofwater vapor or purge gas on the outside of said storage tank.

7. In a cryogenic storage system, as defined in claim 6, including:

a reflective metallic material located on said external surface of thesaid foam coating to form an inner radiation shield.

8. In a cryogenic storage system, as defined in claim 7, including:

' a multi-layer insulation shield located on the outside of said foamcoating to form an outer radiation shield around said storage tank. 5

2. A storage unit, as recited in claim 1, wherein said evacuating meanspurges said control chamber of said low thermal conductive gas and saidcontrol means regulates the flow of helium to said control chamber inresponse to an operator signal for preventing condensation of the lowthermal conductivity gas and water vapor on the outer surface of saidvessel during charging of said vessel with said liquid propellant.
 3. Astorage unit, as recited in claim 2, including: a reflective metallicmaterial plated on a backing member to form a radiation shield, saidradiation shield being wrapped on the outer surface of said foaminsulating member for reducing the transfer of heat to said vessel byradiation from an environmental source.
 4. A method of reducing thetransfer of heat between and external surface of a cryogenic storagetank and the environment, comprising the steps of: coating said externalsurface with a FREON expanded polyurethane foam; surrounding saidsurface with a membrance to form a control chamber therein; purging saidcontrol chamber of air; introducing a gas into said control chamberhaving a relatively low thermal conductivity and relatively highliquidus and solidus temperatures as compared to those of air forinsulating said cryogenic storage tank; wrapping said external surfaceof said layer of foam on the external surface of said storage tank witha reflective metAllic material applied to a backing member to form aninitial radiation shield; and changing said gas to helium gas prior tocharging said storage tank with a cryogen for preventing condensation ofthe said low thermal conductivity gas and water vapor on the outside ofsaid storage tank.
 5. In a cryogenic storage system, means for reducingthe transfer of heat to the external surface of a storage tank,comprising: a foam coating of FREON expanded polyurethane secured to theexternal surface of said storage tank; a flexible membrance surroundingsaid coating having an inlet port and an outlet port, said flexiblemembrane and coating forming a control chamber therebetween; and purgingmeans operatively connected to said inlet port and said outlet port forremoving air from said control chamber and for introducing a gas to saidcontrol chamber having a relatively low thermal conductivity as comparedto that of air and having high liquidus and solidus temperaturesrelative to the usual cryogens for maintaining said FREON expandedpolyurethane in an inert condition.
 6. In a cryogenic storage system, asdefined in claim 5, wherein purging means replaces said gas in saidcontrol chamber with helium prior to charging said storage tank withcryogen for preventing consensation of water vapor or purge gas on theoutside of said storage tank.
 7. In a cryogenic storage system, asdefined in claim 6, including: a reflective metallic material located onsaid external surface of the said foam coating to form an innerradiation shield.
 8. In a cryogenic storage system, as defined in claim7, including: a multi-layer insulation shield located on the outside ofsaid foam coating to form an outer radiation shield around said storagetank.